The present invention relates to a method and apparatus for rapidly controlling the amount of a plating metal deposited in the vapor deposition process for manufacturing a coated steel strip and also relates to an atomic absorption analysis for measuring the amount of a metal vapor carried from a vapor source to a steel strip with high accuracy and high reliability.
In a vapor deposition process line for manufacturing a Zn--coated steel sheet or the like, it is known to measure the amount of a deposited plating metal by fluorescent X-ray spectroscopy or X-ray diffraction techniques.
The use of fluorescent X-ray spectroscopy for this purpose is disclosed in Japanese Patent Application Laid-Open 60-13308. The intensity of fluorescent X-rays emitted from a plating metal by the irradiation of the coated steel sheet with X-rays is measured and compared with a predetermined analytical curve. The amount of deposited plating metal is then calculated from the detected value of the intensity.
According to the fluorescent X-ray spectroscopy technique, an X-ray measuring device 9 shown in FIG. 1 is located at an outlet side of a vapor deposition coating line. A steel strip 1 is carried through a gas reducing furnace 2 wherein the surface of the steel strip 1 is activated. The steel strip is then introduced through inlet hermetic rollers 3 into a vacuum chamber 4. In the vacuum chamber 4, Zn and Mg are vapor deposited to the steel strip 1 in a vapor deposition zone 5. The steel strip 1 is then carried from the vacuum chamber 4 through outlet hermetic rollers 6 and a cooling zone 7.
The X-ray measuring device 9 is located at a position facing the surface of the vapor deposition coated steel strip 8 exiting the vacuum chamber 4. The physical location of the measuring device 9 causes a long time lag in measuring the amount of the deposited plating metal after the vapor deposition. Consequently, operational conditions cannot be quickly controlled in response to changes in the measured deposition amount.
On the other hand, the X-ray diffraction method is suitable for analysis of a coated steel sheet having a multi-layered plating layer. In this method, the peak intensity peculiar to the plating composition is measured, and the deposition amount is calculated by solving the equation relating to the plating composition and the deposition amount.
When different kinds of metals are vapor deposited so as to form a multi-layered plating layer such as a Zn--Mg vapor deposition layer, it is necessary to measure and control the deposition amount of each metal. However, it is difficult to detect the deposition amount of each metal with high accuracy, since X-rays emitted from each sublayer are decayed by the other metal and the other sublayer. In the case of a Zn--Mg plating layer having a multi-layered structure, X-rays emitted from Mg are especially weak and easily decayed by Zn and the alloyed sublayers, making the accurate measurement of the deposition amount fairly difficult.
In addition, in the X-ray diffraction method, the point of measurement is spaced apart from the vapor deposition zone. This space layout location creates long time delays to obtain a measuring result and thus prevents accurate control of the deposition amount.
In the case of a plating layer having a multi-layered structure composed of different kinds of metals including several kinds of intermetallic compounds, it is necessary to measure the peak intensity of each intermetallic compound, using many measuring devices. In addition, since each peak intensity is decayed by the other metal, alloys and sublayers cause measurement errors. Hence, the deposition amount cannot be measured with high reliability in these multi-layered structure.
In addition, the vapor deposition of Zn or Mg onto an inner surface of an apparatus is inhibited by holding said surface at a temperature of several hundreds of degree C, and Zn or Mg vapor is carried from a vapor source through a guide duct to a steel strip. If the amount of a metal vapor passing though the guide duct can be directly measured, the deposition amount of a plating metal is directly controlled with high accuracy.